1. Field of the Invention
The present invention relates to a method for producing a ceramic slurry used for fabricating ceramic electronic components, such as monolithic ceramic capacitors and multilayered ceramic substrates, to a method for forming a ceramic green sheet using the ceramic slurry, and to a method for fabricating a monolithic ceramic electronic component using the ceramic slurry.
2. Description of the Related Art
A monolithic ceramic electronic component, such as a monolithic ceramic capacitor or a multilayered ceramic substrate, is usually fabricated by laminating ceramic green sheets, followed by press-bonding and the performance of a heat treatment to sinter the ceramic and electrodes.
For example, in order to fabricate a monolithic ceramic capacitor as shown in FIG. 1 having a structure in which internal electrodes 2 are arranged in a ceramic element 1 and in which a pair of external electrodes 3a and 3b are arranged on both sides so as to be electrically connected to the internal electrodes 2 which are alternately extended to one side and the other side of the ceramic element 1, the method described below is typically used.
First, a sheet 11 provided with an electrode (refer to FIG. 2) is formed by arranging an internal electrode for forming capacitance on a ceramic green sheet.
Next, as shown in FIG. 2, a predetermined number of sheets 11, each provided with an electrode, are laminated together, and ceramic green sheets (sheets for outer layers) 21 which are not provided with an electrode are laminated on the upper and lower surfaces of the laminated sheets 11, and thus a laminate (press-bonded laminate) is formed in which the ends of the individual internal electrodes 2 alternately extend to one side and the other side.
The press-bonded laminate is then fired under predetermined conditions to sinter the ceramic, and a conductive paste is applied to both sides of the sintered laminate (ceramic element) 1 (refer to FIG. 1), followed by baking, to form the external electrodes 3a and 3b which are electrically connected to the internal electrodes 2.
Thus, a monolithic ceramic capacitor as shown in FIG. 1 can be obtained.
Additionally, other monolithic ceramic electronic components, such as monolithic multilayered ceramic substrates, are also fabricating by laminating ceramic green sheets.
A ceramic green sheet used for fabricating a monolithic ceramic electronic component is generally formed by formulating a ceramic powder, a dispersing medium (solvent), a dispersant, a binder, a plasticizer, etc., at predetermined ratios, followed by mixing and pulverizing with a dispersing machine using a pulverizing medium, such as a bead mill, a ball mill, an ATOLITER, a paint shaker or a sand mill, to form a ceramic slurry, and molding the ceramic slurry into a sheet having a predetermined thickness using a doctor blade process or the like, followed by drying.
However, recently, there have been further demands for reduction in size and higher performance with respect to various monolithic ceramic electronic components, such as monolithic ceramic capacitors, as there are with respect to other electronic elements.
Therefore, ceramic green sheets used for fabricating such monolithic ceramic electronic components must be thinner and very thin ceramic green sheets having a thickness of 10 xcexcm or less have been required.
In the ceramic slurry used for forming such thin ceramic green sheets, the raw material ceramic powder must be sufficiently dispersed. For that purpose, a fine ceramic powder having an average particle size of 0.01 to 1 xcexcm must be used as the raw material ceramic powder.
In the conventional method for forming a ceramic slurry, in which a ceramic powder, a dispersing medium (solvent), a dispersant, a binder, a plasticizer, etc., are formulated at predetermined ratios, and mixing and pulverizing are performed with a dispersing machine using a pulverizing medium, such as a bead mill, a ball mill, an ATOLITER, a paint shaker or a sand mill, it is difficult to sufficiently disperse a fine ceramic powder having particles of 1 xcexcm or less, and it is not possible to obtain a slurry which is homogeneously dispersed. Thus, it is difficult to form thin ceramic green sheets of high quality thereby.
That is, the ceramic green sheet formed using the ceramic slurry produced by the conventional method described above has the problems that 1) the surface thereof is not sufficiently smooth; 2) high density cannot be obtained, resulting in insufficient tensile strength; and 3) resins, such as a binder and a plasticizer, are inhomogeneously dispersed, and the shrinkage factor varies locally in the sintering process after lamination, and thus satisfactory dimensional accuracy cannot be obtained. Additionally, such problems are particularly noticeable when a binder having a high degree of polymerization is used.
In the conventional method for forming a ceramic slurry, in order to improve dispersibility, the ceramic powder may be dispersed by forced impact or collision using a ball mill filled with balls or a bead mill filled with beads. In such a case, the pulverizing force due to impact or collision may be excessively large, thus increasing the level of damage to the ceramic powder, resulting in a decrease in the crystallinity of the ceramic powder or an increase in the specific surface. Thus, it may be difficult to obtain a monolithic ceramic electronic component having desired electrical characteristics.
High pressure dispersion may be used in which a slurry containing a ceramic powder is made to flow under high pressure and the ceramic powder is dispersed by impact or collision. However, since the pulverizing force obtained by the high pressure dispersion alone is smaller than the pulverizing force due to forced impact or collision obtained by the dispersion process using a ball mill or a bead mill, it is difficult to sufficiently pulverize strongly clumped particles, and it is not possible to produce a ceramic slurry which is sufficiently dispersed; thus it is not possible to obtain a ceramic green sheet of high quality.
Even if dispersion treatment is performed in any manner, in the case of a thin ceramic green sheet having a thickness of 10 xcexcm or less, if minute amounts of agglomerated particles, flocculated particles, dust, contamination or bubbles are present, imperfections are generated at the surface of or inside the ceramic green sheet, resulting in problems, such as short circuiting.
The present invention has been achieved in view of the above, and objects of the present invention are to provide a method for producing a ceramic slurry which is suitable for use in fabricating ceramic electronic components, in which a ceramic powder can be homogeneously dispersed without excessive damage and inclusion of foreign matter can be decreased, to provide a method for forming a ceramic green sheet using the ceramic slurry produced by the above method, and to provide a method for fabricating a monolithic ceramic electronic component using the ceramic slurry.
In one aspect, a method for producing a ceramic slurry used for fabricating a ceramic electronic component in accordance with the present invention includes a mixing and pulverizing step for mixing and pulverizing a ceramic powder having an average particle size of about 0.01 to 1 xcexcm, a solvent and a dispersant by a dispersion process using a pulverizing medium, such as balls or beads, to obtain a mixed and pulverized slurry; and a high pressure dispersion step for performing high pressure dispersion at a pressure of about 100 kg/cm2 or more after a filtered binder solution is added to the mixed and pulverized slurry to obtain a dispersed slurry (final dispersed slurry), the filtered binder solution being prepared by dissolving a binder in a solvent, followed by filtration.
It is possible to reliably obtain a ceramic slurry in which the ceramic powder is sufficiently dispersed by the above method in which the ceramic powder having an average particle size of about 0.01 to 1 xcexcm, the solvent, and the dispersant are mixed and pulverized by the dispersion process using the pulverizing medium, such as balls or beads, to prepare the mixed and pulverized slurry, and after the filtered binder solution, which is prepared by dissolving the binder in the solvent, followed by filtration, is added to the mixed and pulverized slurry, dispersion is performed at a high pressure of about 100 kg/cm2 or more.
By dispersing the ceramic powder by combining the dispersion process using the pulverizing medium and the high pressure dispersion process, it is possible to homogeneously disperse the ceramic powder without damaging the crystallinity of the ceramic powder while at the same time inhibiting the specific surface from being excessively increased, and simultaneously, by using the filtered binder solution which is prepared by dissolving the binder in the solvent, followed by filtration, it is possible to reliably remove the undissolved binder, which is likely to generate imperfections, and thus a ceramic slurry suitable for use in fabricating ceramic electronic components, in which occurrence of imperfections is decreased, can be efficiently produced.
In the present invention, xe2x80x9ca filtered binder solution prepared by dissolving a binder in a solvent followed by filtrationxe2x80x9d conceptually includes a solution prepared by dissolving only a binder in a solvent, followed by filtration, and also includes a solution prepared by dissolving a binder together with additives, such as a plasticizer and an antistatic agent, followed by filtration.
In the present invention, xe2x80x9cmixing and dispersing a ceramic powder, a solvent, a dispersant and a binderxe2x80x9d does not mean that the ingredients of the ceramic slurry are limited only to the ceramic powder, the solvent, the dispersant and the binder; the present invention includes cases in which other additives are also added thereto.
Additionally, in the present invention, xe2x80x9ca high pressure dispersion processxe2x80x9d conceptually includes a process in which a slurry is dispersed using a high pressure dispersion apparatus which is constructed, for example, so that the slurry is dispersed by impacting a solution to be dispersed under high pressure on the wall or by passing the solution to be dispersed through a tapered channel.
Although the present invention is particularly useful when the ceramic powder has an average particle size (determined by observation with an electron microscope) of about 0.01 to 1 xcexcm, the present invention is also applicable to cases in which the average particle size is outside the range of about 0.01 to 1 xcexcm.
In another aspect, in accordance with the present invention, a method for producing a ceramic slurry used for fabricating a ceramic electronic component includes a mixing and pulverizing step for mixing and pulverizing a filtered binder solution, a ceramic powder having an average particle size of about 0.01 to 1 xcexcm, a solvent and a dispersant by a dispersion process using a pulverizing medium, such as balls or beads, to obtain a mixed and pulverized slurry, the filtered binder solution being prepared by dissolving a binder in a solvent, followed by filtration; and a high pressure dispersion step for dispersing the mixed and pulverized slurry at a pressure of about 100 kg/cm2 or more to obtain a dispersed slurry (final dispersed slurry).
It is also possible to obtain the same effects as those in the method previously described, by a method in which the filtered binder solution prepared by dissolving the binder in the solvent, followed by filtration, the ceramic powder having an average particle size of about 0.01 to 1 xcexcm, the solvent and the dispersant are mixed and pulverized by the dispersion process using the pulverizing medium, such as balls or beads, to obtain the mixed and pulverized slurry, and then the mixed and pulverized slurry is dispersed at a high pressure of about 100 kg/cm2 or more.
In another aspect, in accordance with the present invention, a method for producing a ceramic slurry used for fabricating a ceramic electronic component includes a mixing and pulverizing step for mixing and pulverizing a ceramic powder having an average particle size of about 0.01 to 1 xcexcm, a solvent and a dispersant by a dispersion process using a pulverizing medium, such as balls or beads, to obtain a mixed and pulverized slurry; a primary high pressure dispersion step for dispersing the mixed and pulverized slurry at a pressure of about 100 kg/cm2 or more to obtain a primary dispersed slurry; and a secondary high pressure dispersion step for performing high pressure dispersion at a pressure of about 100 kg/cm2 or more after a filtered binder solution is added to the primary dispersed slurry to obtain a secondary dispersed slurry (final dispersed slurry), the filtered binder solution being prepared by dissolving a binder in a solvent, followed by filtration.
It is also possible to homogeneously disperse the ceramic powder without excessively damaging the ceramic powder, and thus a ceramic slurry of high quality can be produced by the method in which the ceramic powder, the solvent and the dispersant are mixed and pulverized by the dispersion process using the pulverizing medium to obtain the mixed and pulverized slurry, the mixed and pulverized slurry is subjected to high pressure dispersion (primary high pressure dispersion) at a pressure of about 100 kg/cm2 or more, and high pressure dispersion (secondary high pressure dispersion) is further performed at a pressure of about 100 kg/cm2 or more.
In another aspect, a method for producing a ceramic slurry used for fabricating a ceramic electronic component in accordance with the present invention includes a primary mixing and pulverizing step for mixing and pulverizing a ceramic powder having an average particle size of about 0.01 to 1 xcexcm, a solvent and a dispersant by a dispersion process using a pulverizing medium, such as balls or beads, to obtain a primary mixed and pulverized slurry; a secondary mixing and pulverizing step for mixing and pulverizing by a dispersion process using a pulverizing medium, such as balls or beads, after a filtered binder solution is added to the primary mixed and pulverized slurry, to obtain a secondary mixed and pulverized slurry, the filtered binder solution being prepared by dissolving a binder in a solvent, followed by filtration; and a high pressure dispersion step for dispersing the secondary mixed and pulverized slurry at a pressure of about 100 kg/cm2 or more to obtain a dispersed slurry (final dispersed slurry).
It is also possible to homogeneously disperse the ceramic powder without excessively damaging the ceramic powder, and thus a ceramic slurry of high quality can be produced by the method in which the ceramic powder, the solvent and the dispersant are mixed and pulverized by the dispersion process using the pulverizing medium to obtain the primary mixed and pulverized slurry, after the filtered binder solution is added to the primary mixed and pulverized slurry, mixing and pulverizing are performed again to obtain the secondary mixed and pulverized slurry, and the secondary mixed and pulverized slurry is dispersed at a high pressure of about 100 kg/cm2 or more.
In the method for producing a ceramic slurry in the present invention, the filtered binder solution may be prepared by mixing the solvent and the binder, and performing high pressure dispersion at a pressure of about 100 kg/cm2 or more, followed by filtration.
By using the filtered binder solution prepared by mixing the solvent and the binder, and by performing high pressure dispersion at a pressure of about 100 kg/cm2 or more, followed by filtration, gels which may be generated when the binder is directly added or when the binder dissolved in the solvent is added without filtration, can be prevented from occurring, and thus the dispersibility of the ceramic powder can be further improved.
In a method for producing the ceramic slurry in the present invention, the filtered binder solution may be prepared by refluxing a binder solution comprising a mixture of the solvent and the binder at about 40 to 100xc2x0 C., followed by filtration.
By using the filtered binder solution prepared by refluxing the binder solution comprising the mixture of the solvent and the binder at about 40 to 100xc2x0 C., followed by filtration, the binder can be more reliably dissolved, and the binder can be added without generating imperfections or micrometer-size agglomerates, and it is also possible to improve the dispersibility of the ceramic powder.
In a method for producing the ceramic slurry in the present invention, the filtered binder solution may be prepared by filtering at a filtration cutoff accuracy of about 99% using a filter having pores having a diameter of about 2 xcexcm or less.
By using the filtered binder solution prepared by filtering at a filtration cutoff accuracy of about 99% using the filter having pores having a diameter of about 2 xcexcm or less, it is possible to reliably remove the undissolved binder, thus making the present invention more effective.
Herein, xe2x80x9ca filtration cutoff accuracy of about 99%xe2x80x9d means that about 99% or more of particles having diameters greater than the predetermined value of filtration cutoff accuracy are captured by a filter, and for example, a method according to the single pass F-2 test based on ANSI B9331-1973 may be mentioned. xe2x80x9cFiltering at a filtration cutoff accuracy of about 99% using a filter having pores having a diameter of about 2 xcexcm or lessxe2x80x9d means that filtration is performed at a filtration cutoff accuracy of about 99%, at a filtration level of about 2 xcexcm or less.
Examples of materials for the filtration film are metals, PTFE, polypropylene and nylon. However, the material for the filtration film is not limited thereto.
As filtration elements, such as a filtration film, used herein, for example, a sheet-type element referred to as a xe2x80x9cmembranexe2x80x9d, an element referred to as a xe2x80x9csurfacexe2x80x9d on which a membrane is arranged, and an element referred to as a xe2x80x9cdepthxe2x80x9d in which a material for a filtration film which is shaped like a thread is wound, may be mentioned. However, the filtration element is not limited thereto.
A filter having one level of filtration cutoff accuracy may be used or a plurality of filters having different levels of filtration cutoff accuracy may be used in sequence. However, the specific use of the filter is not particularly limited.
In the method for producing the ceramic slurry in the present invention, preferably, the ceramic slurry (final dispersed slurry) has a viscosity of about 0.003 to 0.1 Paxc2x7s.
If the viscosity of the dispersed slurry (final dispersed slurry) is set at about 0.003 to 0.1 Paxc2x7s, it is possible to produce a ceramic slurry which is suitable in use for molding into a sheet to produce a ceramic green sheet, thus making the present invention more effective.
Additionally, although the lower viscosity is suitable for forming thinner green sheets, if the viscosity is less than about 0.003 Paxc2x7s, the shape retaining ability is deteriorated and variation in sheet thickness occurs, and thus the viscosity is preferably set in the range of about 0.003 to 0.1 Paxc2x7s.
In the method for producing the ceramic slurry in the present invention, preferably, the dispersion process using the pulverizing medium is a process employing either a ball mill or a bead mill.
If the ball mill or the bead mill is employed in the dispersion process using the pulverizing medium, flocculated ceramic particles can be reliably pulverized, thus making the present invention more effective.
Additionally, in the dispersion process of the present invention, a dispersing machine using a pulverizing medium, such as an ATOLITER, a paint shaker or a sand mill, may be used instead of the ball mill or bead mill.
In the method for producing the ceramic slurry in the present invention, preferably, an anionic dispersant is used as the dispersant and the amount thereof to be added is set so that the total acid amount of the anionic dispersant is about 10 to 150% of the total base amount of the ceramic powder.
Examples of the anionic dispersant which can be preferably used in the present invention are anionic dispersants containing carboxylates, maleates, sulfonates, phosphates, etc. Examples of the anionic dispersants which can be more preferably used are polycarboxylic acid-type dispersants and polymaleic acid-type dispersants which do not contain metallic ions.
With respect to the content of the anionic dispersant, preferably, the total acid amount of the anionic dispersant is about 10 to 150% of the total base amount of the ceramic powder. If the total acid amount of the anionic dispersant is less than about 10% of the total base amount of the ceramic powder, a satisfactory dispersion effect is not displayed, and even if it exceeds about 150%, the dispersion effect is not significantly improved.
Additionally, the total acid amount of the anionic dispersant and the total base amount of the ceramic powder may be determined by titration or the like.
In the method for producing the ceramic slurry in the present invention, preferably, the ceramic slurry (final dispersed slurry) is used for forming a ceramic green sheet and a material having a particle size greater than the thickness of the ceramic green sheet to be formed is prevented from being present in the ceramic slurry.
By preventing the material having a particle size greater than the thickness of the ceramic green sheet to be formed from being present in the ceramic slurry (final dispersed slurry), when the ceramic green sheet is formed by molding the ceramic slurry into a sheet, it is possible to reliably obtain a ceramic green sheet having superior smoothness and uniformity without protrusions or the like on the surface.
Additionally, xe2x80x9ca material having a particle size greater than the thickness of the ceramic green sheet to be formedxe2x80x9d means a material in which any one of the maximum length, the maximum thickness and the maximum width exceeds the thickness of the ceramic green sheet. Such a material conceptually includes, for example, a gel substance in addition to a solid and a crystalline substance. Specific examples thereof include dust and contamination in air, an agglomerate composed of a plurality of ceramic particles generated when dispersion is insufficient or generated when dispersed particles are agglomerated again after dispersion.
In order to prevent the material having a particle size greater than the thickness of the ceramic green sheet from being present in the ceramic slurry, such a material may be separated out in advance by a separation method, e.g., filtration or centrifugal separation, or the ceramic slurry may be prepared by formulating the individual ingredients in the state in which such a material is not present.
In another aspect, a method for producing a ceramic slurry in accordance with the present invention includes the step of filtering the ceramic slurry produced by any one of the methods for producing the slurry described above, at a filtration cutoff accuracy of about 99% using a filter having pores having a diameter less than about 5 times the thickness of the ceramic green sheet.
By filtering the ceramic slurry at a filtration cutoff accuracy of about 99% using the filter having pores having a diameter less than about 5 times the thickness of the ceramic green sheet, when the ceramic slurry is molded into a sheet to produce the ceramic green sheet, it is possible to prevent protrusions from occurring in the surface and to avoid decreases in smoothness and uniformity due to the inclusion of a material having a particle size greater than the thickness of the ceramic green sheet to be formed, and a ceramic green sheet having superior smoothness and uniformity can be reliably obtained.
If a material having a particle size greater than the thickness of the ceramic green sheet is present in the ceramic slurry, the material may protrude from the surface of the ceramic green sheet, or the material may remain in the ceramic green sheet, resulting in defects such as short circuiting. However, filtration at a filtration cutoff accuracy of about 99% using the filter having pores having a diameter less than about 5 times the thickness of the ceramic green sheet makes it possible to reliably remove such a material.
Herein, xe2x80x9ca filtration cutoff accuracy of about 99%xe2x80x9d means that about 99% or more of particles having sizes greater than the predetermined value of filtration cutoff accuracy are captured by a filter.
xe2x80x9cFiltration at a filtration cutoff accuracy of about 99% using a filter having pores having a diameter less than about 5 times the thickness of the ceramic green sheetxe2x80x9d means that, for example, when the ceramic green sheet has a thickness of 2 xcexcm, filtration is performed at a filtration cutoff accuracy of 99%, at a filtration level of 10 xcexcm or less.
Additionally, in the present invention, filtration at a filtration cutoff accuracy of about 99% using a filter having pores having a diameter less than the thickness of the ceramic green sheet is not essential, and filtration at a filtration cutoff accuracy of about 99% using the filter having pores having a diameter less than about 5 times the thickness of the ceramic green sheet is sufficient. The present inventor has confirmed from various tests repeatedly conducted, that substantially all the materials having a particle size greater than the thickness of the ceramic green sheet can be removed by filtration at a filtration cutoff accuracy of about 99% using a filter having pores having a diameter less than about 5 times the thickness of the ceramic green sheet.
Additionally, the filtration cutoff accuracy of about 99% is preferably from about 3 times the average particle size of the ceramic powder to about 3 times the thickness of the ceramic green sheet. Thereby, it is possible to reliably remove materials having a particle size greater than the thickness of the ceramic green sheet and the time required for the filtration process can be decreased, thus improving productivity.
Examples of a material for the filtration film are metals, PTFE, polypropylene and nylon. However, the material for the filtration film is not limited thereto. With respect to filtration elements, for example, a sheet-type element referred to as a xe2x80x9cmembranexe2x80x9d, an element referred to as a xe2x80x9csurfacexe2x80x9d on which a membrane is arranged, and an element referred to as a xe2x80x9cdepthxe2x80x9d in which a material for a filtration film which is shaped like a thread is wound, may be mentioned. However, the filtration element is not limited thereto.
A filter having one level of filtration cutoff accuracy may be used or a plurality of filters having different levels of filtration cutoff accuracy may be used in sequence. However, the specific use of the filter is not particularly limited.
Additionally, in a more preferable example, first, a depth-type filter is used as a first filter and then a membrane-type or surface-type filter is used as a second filter, in which the level of filtration cutoff accuracy of the second filter is set higher than that of the first filter. Thus, in the primary filtration process in which the amount of material to be collected per filter is greater, most of the materials are removed by the depth-type filter with high volume treating capacity, and in the secondary filtration process in which the amount of material to be collected is small, it is possible to perform filtration with higher accuracy using the membrane-type or surface-type filter having the higher level of filtration cutoff accuracy.
In another aspect, a method for forming a ceramic green sheet having a thickness of about 0.1 to 10 xcexcm in accordance with the present invention includes the step of molding the ceramic slurry produced by any one of the methods described above into a sheet on a predetermined base.
In the ceramic slurry produced by any one of the methods described above, a ceramic powder having an average particle size of about 0.01 to 1 xcexcm is sufficiently dispersed in the dispersing medium, and by molding the ceramic slurry into a sheet, it is possible to reliably form a thin ceramic green sheet of high quality (about 0.1 to 5 xcexcm thick). That is, it is possible to obtain a ceramic green sheet having superior surface smoothness, high density and high tensile strength, in which resins, such as a binder and a plasticizer, are homogeneously dispersed and which is suitable for use in fabricating a monolithic ceramic electronic component. When a monolithic ceramic electronic component is fabricated using the ceramic green sheet, it is possible to obtain a highly reliable monolithic ceramic electronic component of high quality having desired characteristics.
In another aspect, a method for fabricating a monolithic ceramic electronic component in accordance with the present invention includes the steps of forming ceramic green sheets using a ceramic slurry produced by any one of the methods described above, laminating the ceramic green sheets together with internal electrodes composed of a base metal, followed by dicing and firing, and forming external electrodes.
By forming ceramic green sheets using the ceramic slurry produced by the method of the present invention, laminating the ceramic green sheets together with internal electrodes composed of a base metal, followed by cutting and firing, and forming external electrodes, it is possible to obtain a reliable monolithic ceramic electronic component of high quality having desired characteristics.